Update a few more methods in FixProcessor, and fix some bugs in ChangeBuilder

pkg/analysis_server/test/services/correction/fix_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/edit/fix/fix_core.dart';
 import 'package:analysis_server/plugin/edit/fix/fix_dart.dart';
 import 'package:analysis_server/src/services/correction/fix.dart';
 import 'package:analysis_server/src/services/correction/fix_internal.dart';
@@ skipping 3412 matching lines @@
 library main;
 int main() async {
 }
 ''');
     await assertHasFix(
         DartFixKind.REPLACE_RETURN_TYPE_FUTURE,
         '''
 library main;
 
 import 'dart:async';
+
 Future<int> main() async {
 }
 ''');
   }
 
   test_illegalAsyncReturnType_asyncLibrary_usePrefix() async {
     errorFilter = (AnalysisError error) {
       return error.errorCode == StaticTypeWarningCode.ILLEGAL_ASYNC_RETURN_TYPE;
     };
     await resolveTestUnit('''
@@ skipping 3459 matching lines @@
   @override
   final AnalysisError error;
 
   _DartFixContextImpl(this.resourceProvider, this.analysisDriver,
       this.astProvider, this.unit, this.error);
 
   @override
   GetTopLevelDeclarations get getTopLevelDeclarations =>
       analysisDriver.getTopLevelNameDeclarations;
 }